1. Field of the Invention
This invention relates in general to storage devices and in particular to a holographic data storage wherein a single scanning beam is utilized to read information from a plurality of holographic storage plates.
2. Description of the Prior Art
As computers have become more and more common, the requirement for greater storage capacity and more rapid access to information has become desirable. Storage in holographic form has been proposed which makes it possible to store information in greater density than that possible with prior storage techniques, as for example, magnetic storage. It has been know to spacially modulate coherent light and to store the data holographically. In such storage devices, any interference pattern can be assigned one unit of binary data or one bit. The presence of the pattern corresponds to the binary 1, and its absence to the binary 0.
With the aid of holograms, it is possible to project images to arbitrary positions in space without the assistance of optical elements. Therefore it is possible to construct holographic storage devices with non-mechanical, random access which can have very large capacities. The use of holographic storage techniques overcomes tolerance problems such as occur in other high capacity optical storage devices since the data output is obtained by interrogating an image with a relatively crude structure located outside the hologram.
Each holographically stored bit corresponds to a single spot on an object surface whose associated interference field is uniformly stored on the entire hologram surface. Holographic storage is substantially immune to dust, local scratches and inhomogeneities of the storage medium, since local disturbances on the hologram result merely in a slight reduction in the signal to noise ratio. The spots are arranged as a grid pattern; and if this spot grid pattern is coherently illuminated from the side facing away from the hologram surface, the binary numbers can be recorded in parallel form onto the hologram.
If the data stored in such structure are to be read out in parallel, a special photo-detector will be required for each bit. Due to cost consideration, it will normally be prohibitive to emit the entire contents of a large capacitive storage device in parallel.